The study on distribution of substance P in brain indicated that this peptide is most concentrated in substantia nigra and interpeduncular nucleus. By lesion studies, the substance P containing cells bodies which innervate substantia nigra and interpeduncular nucleus were identified to be located in striatum and habenulla, respectively. The chronic treatments with haloperiodol lead to a 50 percent decrease in substance P concentration in substantia nigra. This action of haloperidol appears to be expressed exclusively in the substance P content of substantia nigra. The result suggests a possible interaction between dopaminergic neurons and substance P containing neurons in substantia nigra. BIBLIOGRAPHIC REFERENCES: Hong, J.S., Costa, E. and Yang, H.-Y. T: Effects of habenular lesions on the substance P content of various brain regions. Brain Res. 118: 523-525, 1976. Hong, J.S., Yang, H.-Y. T., Racagni, G. and Costa, E.: Projections of substance P containing neurons from neostriatum to substantia nigra. Brain Res. 122: 541-544, 1977.